Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Micromirror devices are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and micromirrors have found commercial success, other types have not yet been commercially viable.
Micromirrors primarily are used in optical display systems. In display systems, the micromirror is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, micromirrors typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the micromirror surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
While there has been a rapid advance in the complexity and function of the micromirror array and a corresponding improvement in micromirror-based display systems, there are several micromirror manufacturing processes that continue to add a great deal of expense to the micromirror. In particular, some types of micromechanical devices are extremely sensitive to particles. Unfortunately, this particles are very difficult to remove since the devices are very fragile and cannot be exposed to typical clean-up processes such as a water or air scrub. Because the wafer separation process generates many of the particles, the devices are sometimes separated and washed prior to completing the device fabrication. This forces the remaining processing steps to be completed on individual devices—greatly driving up the cost of processing.
One process used to manufacture micromirror devices separates a wafer into individual die and mounts the die in the device package. After the die has been mounted, it is undercut removing the sacrificial layers of photoresist on which the mirrors have been formed. While this improves the manufacturing yield of the devices since the particle risk is greatly reduced, it does not realize the full cost savings of the higher yield since the nonfunctional parts are mounted in the fairly expensive package prior to testing.
What is needed is a process that enables micromechanical devices to be fully tested prior to wafer separation, yet enables the devices to be protected.